1. Field of the Invention:
This invention relates to a frequency divider, and particularly to a circuit for dividing a pulse frequency by a number obtained by dividing an odd number by two.
2. Description of the Prior Art:
A frequency divider using a programmable counter has been used in various apparatus such as a Phase Locked Loop (PLL) frequency synthesizer. As a means for improving a Carrier Noise Ratio (CNR) in such a PLL frequency synthesizer, a frequency divider dividing pulses by a number obtained by dividing an odd number by two is successfully employed.
An example of a known frequency divider has a counter made of a plural number of series-connected flip-flops counting input pulses and a means for changing a phase of the input pulses in response to an output from the counter obtained at a time when the counting value becomes a predetermined number. The phase changing means consists of a first AND circuit directly receiving input pulses from an input terminal, a second AND circuit receiving the input pulses through an inverter, an OR circuit receiving outputs from the first and second AND circuits and a flip-flop producing, in response to the output from the counter, an output to be applied to the first AND circuit and an inverted output to be applied to the second AND circuit. In accordance with the output and the inverted output from the flip-flop, one of the first and second AND circuits passes the input pulses or the inverted input pulses, and thus the change of phase of the input pulses is achieved.
Since the first and second AND circuit pass the input pulses and inverted input pulses in accordance with the output and the inverted output from the flip-flop, the timings of the input (or inverted input) pulses and the output (or inverted output) from the flip-flop are important to achieve a normal operation. However, when the input pulses become high frequency, a mismatching of their timings is apt to occur and destroys the normal operation. Therefore, the frequency divider in the prior art has a limit in operable frequency of the input pulses.